depends on where you live and how you dress
the pocket 380 is the minimum for SD so most people say
and its so light the pocket carry with a holster should be with you always
the 9's are harder to conceal so people use a vest or a wrap of some kind
not an issue with the pocket carry
the 9mm ammo is a lot cheaper
there are several good small 9mm manufacturers out there

Since you settled on the 9mm be sure and use high quality rounds designed for self defense.Federal,winchester and others produce quality rounds.Look at what the federal agents and large police depts use.These are choices based on lab and real world shootings.Although some are limited to LE only there are similiar rounds for civilians.You want a round above 1000fps,able to penetrate 12-15 inches and reliable expansion to somewhere around .60 diameter.Cost shouldnt be a concern on a primary carry round.This is the last place to scimp.Choices are a personal decision but look at the data out there on quality self defense rounds.Even experts dont totally agree but all seem to agree on some basic theories.There are no majic bullets but quality rounds along with sufficient numbers in a important vital area will stop the aggression.
The 9mm is a great round with proven performance.Now this part is just what I do.For practice I use a cheaper round that has closely the same characteristics.I do this as my primary carry round and range round perform pretty much the same.Be sure to fire primary carry to see how it performs.There should be no failures.Good to find out what went wrong at the range instead of when it is needed.Do some testing of your own.Use medium such as wet phonebooks,old car doors and homemade ballistics gel.There are recipes out there for gel that closely replacates 20% ordnance jell.It just requires a little work and it is fun to shoot.

I have ben doing A LOT of computer research, visiting LGSs, and shows recently since I am retired (lot of time available).
the only 380 I would get is the sig 238 - it a locked breech instead of a blow back
that ruger lcp tore my hand and my friends hand up with recoil - didn't get 100 rounds thru before quiting
I sold it
more and more people are finding and posting on the same thing i found
was thinking about the beretta cheetah or bersa thunder plus to have a large capacity gun but it is so hard to pull the slide because of them being a blowback design i gave up on them
I found a great thread on a different forum - with many replies - comparing the plastic 380 vs the sig steel locked breech
locked breech handguns are the way to go

don't think there are many blow back 9mms out there - just too much power
the new single stack 9s are wonderful but it seems from all my reading and my recent posted poll that the S&W Shield is the best single stack 9mm with that wonderful trigger
however IMO I want a pocket pistol so the sig 938 is my choice
I recently held one - by surprise i might say - thought it would be a long while before I saw one
it is just a bit bigger than the 238 sig but big enough to really make a difference

It's cute to say "oh i have big nuts and carry a 9mm/ .40 for self defense"
I have a .40 cal CCW G27 that i usually carry over my .380 S&W Bodyguard

BUT!!!! The .380 goes where the G27 doesn't/ can't... Consider that also 99.999% of the time you won't be drawing the gun anyways and itll just stay in your pocket. I will say I advocate having both a 9/40 ccw AND a .380 because you will be in better positioning with a 9/40, but the .380 will suffice

Re: CCW - .380 or 9mm

I feel sticking with 9mm would be a good choice, for the little bit of size difference, you most of the time get a gun more shooter friendly. I'm a big guy and I carry a xdm most the time, in a crossbreed supertuck and love the set up. I like the beretta nano, and Sig p938. Some odd reason the Sig just make me want to smile when I see them.